Injection-blow molding apparatus with parison heat redistribution means

ABSTRACT

Method and apparatus for molding plastic articles by forming a parison on a core within an injection mold, cooling the internal and external walls of the parison thus formed by heat transfer through the core and the injection mold, transferring said core to a delay position for permitting redistribution of heat from the interior of the parison to the exposed walls to obtain a more uniform temperature just above the material&#39;s glass transition temperature, and subsequently transferring said core to an expansion mold for expanding same, the cooling and heat redistribution permitting a greater degree of orientation of the polymer chains of the formed articles. Preferably, the apparatus for practicing the process includes injection and expansion molds supported in such a relation as to permit a substantial time interval between injection and expansion of the parison.

This is a division of Ser. No. 201,823, filed Nov. 24, 1971.

BACKGROUND OF THE INVENTION

This invention relates to the field of injection blow molding. Morespecifically, it relates to a method and apparatus for obtaining ahigher degree of biaxial orientation of articles made by such processes,and may have application in the field of either blow molding or vacuummolding. Prior designs for injection blow molding machines haveprimarily related to designs in which minimum machine cycle time couldbe obtained. For example, U.S. Pat. No. 2,853,736 teaches the formationof a parison at a first injection station and immediate transfer thereofto a second station at which they are blown to desired configurationwith subsequent ejection of the blown article at a third station. Whilesuch apparatus may be sufficient and economical for the manufacture ofmany articles, they are not believed to result in a high strength,oriented article such as a bottle suitable for containing pressurizedliquids or beverages.

Further, U.S. Pat. No. 3,492,387 suggests that a thermoplastic materialmay be injected into an injection mold; that the slug of thermoplasticmaterial is to be held in the mold for a time period of four seconds topermit cooling, with subsequent expansion of the thermoplastic slug bymechanical means; such procedures intended to result in a more highlyoriented hollow article. Finally, U.S. Pat. No. 3,311,684 suggests thata parison may be extruded, with the interior walls of said parison beingcooled by a fluid, followed by stretching of the parison in vertical andin radial directions so as to obtain the desired biaxial orientation.

Although U.S. Pat. No. 3,492,387 teaches orientation, it is not believedthat the apparatus is best suited for the formation of containers forpressurized liquids or achieves the desired strength. Alternatively,U.S. Pat. No. 3,311,684 involves an extrusion process with its resultingwaste, and does not seemingly provide for uniform heat distributionwithin the parison whereby maximum orientation can be achieved.

SUMMARY OF THE INVENTION

In order to overcome these disadvantages and to produce a containerhaving a high degree of biaxial orientation of the polymeric chains ofthe thermoplastic material, and to provide a container suitable forholding pressurized liquids, the instant process and apparatus forpracticing same first utilizes an injection process in which athermoplastic material is injected into an injection mold about a core,with subsequent cooling of the internal and external surface walls ofthe parison thus formed. Inasmuch as such cooling primarily takes placeat or near these surfaces, the entire parison is not reduced to atemperature just about its glass transition temperature (T_(G)) suchthat maximum orientation of the article can be achieved. Accordingly,this invention includes an additional step of tranferring the parison toa holding station whereby the heat within the parison interior may beredistributed to achieve a more uniform temperature (just above T_(G)for the material) throughout a cross-section of the parison wall withsubsequent expansion thereof so as to more completely align thepolymeric chains. Too, such expansion process also includes theutilization of a mold having substantial heat transfer capabilitieswhereby the biaxial orientation may be "frozen into" the formed article.Similarly, the apparatus of the instant invention includes a corecarrier, and an injection and expansion mold so arranged at stationlocations that a holding cycle or station exists between the formingstep and the expansion step. Such arrangement is capable of permittingthe formation of oriented containers at the highest possible productionrate.

Accordingly, it is an object of the instant invention to provide aprocess for manufacturing hollow articles suitable for containingpressurized liquids. Too, it is a further object of the instantinvention to provide a process whereby the maximum biaxial orientationof hollow articles may be obtained and to utilize the inherent heatconduction properties of thermoplastic so as to obtain a parison havingmore uniform temperature throughout its cross-section for this purpose.Finally, apparatus is suggested whereby such orientation may beaccomplished without reducing the production rate of the formedcontainers so as to permit an economical process.

DESCRIPTION OF THE DRAWINGS

The manner in which the objects of the invention are attained will bemade clear by a consideration of the following specification and claimswhen taken in conjunction with the drawings in which:

FIG. 1 is a side elevational view of the apparatus which may be utilizedin practicing the instant process;

FIG. 2 is a top view of a turntable which is utilized to carry the coreson which the parison are formed; and

FIG. 3 is an elevational view partially in section of a core utilized inpracticing the instant invention.

DETAILED DISCUSSION

As previously indicated, the instant invention includes the injection ofa thermoplastic material about a core within an injection mold to form aparison and subsequent heat transfer from the parison's exposed walls toa fluid circulating within a core and the injection mold. Subsequently,the parison is transferred to a holding station whereby heatredistribution within the parison results in a more uniform parisoncross-section temperature just above its glass transition temperature.Then, the parison is transferred to an expansion mold whereby a fluidenergy is exerted against the interior wall of the parison causing sameto expand against the cavity walls to form a completed article. Too, bycontrolling the temperature within the expansion mold, the orientationof the polymer chain is "locked in".

The instant process will enhance the orientation of the polymeric chainsof most thermoplastic resins used for injection mold applications.However, preferred resins are those of thermoplastic polymers having ahigh nitrile monomer content in the order of 60% or more such as thecopolymers of olefinically unsaturated nitriles such as acrylonitrile,methacrylonitrile and ethylenically unsaturated comonomers such as alkylacrylates, styrene and graft copolymers of the nitrile copolymers withdiene rubbers.

The process is generally indicated in FIGS. 1 and 2 which depict aparison 5 being injected through nozzle 4 of an extruder (not shown)into an injection mold 15 about a core 52 at station A. Preferably, foursuch cores 52 are carried by turntable 40 by radially extending arms 42at 90 degree intervals. After the parison 5 is formed in the injectionmold, it is held in such position for approximately four or five secondsto permit cooling of the material adjacent external and internal walls 7and 8 (see FIG. 3), after which time the turntable 40 is rotated 90° toa cooling station B. At this station, heat transfer from the interior 9of the parison to the surface walls 7 and 8 may be effected. After thetemperature throughout a cross-section of the parison wall is permittedto become more uniform, the turntable 40 will again be rotated another90 degrees to station C in juxtaposition with the expansion mold 25.This mold is then closed, with air being emitted through the core 52 toexpand the parison against the cavity walls thereof which are cooled bythe passage of fluid therethrough so as to lock in the orientation. Thecore is then indexed on the turntable 40 another 90 degrees to anejection station D at which point, ejection means 32 will strip theformed article from the core.

FIG. 1 indicates appropriate movement of the turntable. This sideelevational view depicts the turntable 40 as being supported for bothvertical and rotational movement, and appropriate motors foraccomplishing such movement are well known in the art. Specifically,U.S. Pat. No. 3,100,913 which issued to De Matteo on Aug. 20, 1963depicts one such arrangement. The lower mold halves 17 and 27 of theinjection mold and the blow mold are carried by a support 11, while theupper mold halves 16 and 26 are moved vertically as indicated byhydraulic motors (not shown). Prior to indexing the table 40 by anappropriate motor (not shown) the upper mold halves 16 and 26 must bemoved to the dotted line position, while the turntable is elevatedslightly by a motor (not shown) so as to permit the parison 5 in theinjection mold 15 and the formed article 6 in the expansion mold 25 toclear the lower mold halves at which time, rotation is effected.

Thus in operation, the molds 15 and 25 are initially open, and theturntable 40 is rotated so as to align two of the cores 52 injuxtaposition with the molds 15 and 25. Simultaneously, the turntable 40and the upper mold halves 16 and 26 are lowered with two of the cores 52resting in the lower mold halves with the upper mold halves closing overthem. Accordingly, injection and expansion will take place in these twomolds 15 and 25 while cooling of another parison and ejection of aformed article occurs at the other cores 52. Subsequently, the uppermold halves are opened with the turntable rising sufficiently to clearthe lower mold halves and indexing again is effected.

Thus, in considering FIGS. 1 and 2 together, it should be apparent thatthe turntable 40 will successively rotate each of the cores 52 to theinjection mold 15, the holding station B, the blow mold 25, and thebottle ejection station D. Too, it should also be apparent that the timelapse which occurs at the cooling station is substantially equal to thetime for closing the molds 15, injecting a thermoplastic material intosame, the cooling time permitted in the injection mold and the timerequired for rotation. By providing such additional cooling period atstation B, redistribution of heat within the parison walls will beaffected as more fully described hereinafter.

In order to maximize the production rate of formed articles, rapid anduniform cooling of the parison is desired. Accordingly, substantialcooling of the interior and exterior walls 7 and 8 of the parison 5 isobtained within the injection mold, with the uniformity occurring as aresult of heat redistribution at the cooling station B.

As shown in FIG. 1, the exterior wall 7 of the parison may be cooledthrough the utilization of cooling passages 20 within the injectionmold, which are well known in the art. Thus, fluid may be directedthrough conduits 18, passages 20 and exhausted from the opposite end ofthe injection mold by conduits 19.

FIG. 3 depicts structure for the cores 52 which may be utilized to coolthe interior wall 8 of the parison. The radial arm 42 mounted onturntable 40 carries the cylindrical core 52 as shown. Extending throughthe arm 42 and the core 52 is a bore 57 which carries a control rod 54.As depicted, the control rod is a cylindrical member having at itsforward end a conical end section 58 which sealingly engages a conicalcounter bore 53 on the end of the core 52. Within the arm 42 is acounter bore 68, into which the control rod 54 extends and is threadedlyengaged with an annular sealing member 72. A spring 76 interposedbetween the sealing member 72 and the end of the counter bore 68 urgesthe control rod 54 to the rear or left as viewed in the drawing.

As previously indicated, heat is to be transferred from the interiorwall 8 of the parison, through the core 52 by the utilization of fluidpassages. Such fluid passages may take the form of double threads 56located 180 degrees apart on the exterior wall of the control rod 54.Accordingly, fluid may be entered through a bore 51 in the arm 42 andtravel through one of the threaded grooves to the forward end of thecontrol member and return through the opposite groove and out anaperture 50 on the opposite side thereof. Appropriate seals as shown onthe opposite ends of the control rod may be used to effectively precludethe leakage of fluid from the core.

In order to cause expansion of a parison previously formed on the core52 within the blow mold, a bore 59 is formed within the control rod 54.At the forward end of the control rod is a cross drill 60 which permitsair flowing into the bore 59 to pass between the surfaces formed by theend section 58 and conical bore 53 when the control member is urgedforward. Such air is admitted to the bore via an aperture 61 in arm 42and cross drill 62 within control member 54. As is well known in theart, camming means may be utilized to urge the control member 54 forwardat the appropriate time so as to emit air to the interior of the parisonand expand same against the walls of the expansion mold.

MODE OF OPERATION

Assuming that a core 52 has been rotated in juxtaposition the injectionmold 15 with this mold being subsequently closed, a thermoplasticmaterial may be injected into the mold about the core through the nozzle4. The time period for injection and the holding of pressure within theinjection mold may take three of four seconds. Subsequently, the nozzle4 is retracted in the conventional manner and the surfaces 7 and 8 ofthe parison are then cooled for several additional seconds. Ideally, itis contemplated that the surface of the core 52 should be maintained atthe glass transition with the injection mold being maintained at a lowertemperature through heat transfer to fluid passing through the core 5and the injection mold 15. Thus, during the delay period within theinjection mold, the interior 7 and exterior 8 walls of the parison andthe thermoplastic adjacent thereto are rapidly cooled to or below anorientation temperature. (Due to the high thermal conductivity of themetal core and injection mold, the surfaces 7 and 8 are believed toachieve substantially the same temperature as those metal components.)With respect to the cooled surface walls, the interior 9 may be regardedas a heat reservoir.

Subsequently, the upper mold half is opened, the turntable 40 raised androtated as previously indicated so as to move the cores 52 and formedparison 5 to a holding station B. At this holding station, heat willcontinue to flow from the interior wall 8 of the parison through thecore 52 and the thermoplastic material in this region will be reduced toa point just above its glass transition temperature. Simultaneously,however, heat will flow from the interior 9 of the parison outwardly soas to reheat the exterior wall 7 of the parison to a point just aboveits glass transition temperature. Accordingly, this heat transferprocess results in a more uniform temperature of the cross-section ofthe parison wall in a minimum amount of time. From the holding stationB, the parison is indexed to the blow mold for expansion, such beingaffected by the delivery of air through the bore 61 of the arm 42, intothe cross drill 62, bore 59, cross drill 60 and out of the end of thecore the forward movement of the control rod being affected by cams (notshown) so as to open the seal effected between the surfaces of thecounter bore 53 and the conical end 58 of the control member.Simultaneously with such expansion, the expanded article will be cooledby fluids passing through passages 29 within the expansion mold 45 so asto freeze or lock in the orientation of the polymer chains. Subsequentopening of the mold 45 and rotation of the turntable will move theformed article on its core to an ejection station at which point thebottle or container may be removed.

As contemplated, it is anticipated that during each of the four cycles,or one complete revolution of the turntable, four complete articles willbe made. As viewed in FIG. 1, it is also anticipated that several moldsmay be vertically stacked one upon the other, or they may be placed sideby side so as to increase the maximum rate of production.

EXAMPLE

Containers fabricated from Barex 210 a commercially available amorphousthermoplastic nitrile resin consisting ofacrylonitrile/methylacrylate-acrylonitrile/butydiene graft copolymercontaining about 69% acrylonitrile, 22% methylacrylate, and 9% butadienerubber were formed through the process herein identified. This resin hasa glass transition temperature of approximately 180°F, and was injectedinto an expansion mold of the type illustrated in FIG. 1 in aplasticized state, at a temperature of approximately 390°F. The fluidpassing through the injection mold 15 was maintained at a temperature of120°F while a coolant within the blow stick 52 was maintained atapproximately 200°F, approximately 20°F above the glass transitiontemperature of Barex 210. The extruder device injected and held apressure within the injection mold 15 for a period of three seconds,while cooling of the parison against the surfaces of the core and theinjection mold continued for an additional 4 seconds. The core was thentransferred to a holding station, with a total delay time (includingtime for movement of the core and time at station B) of twelve secondsbefore actual blowing of the container within a blow mold having acoolant passing therethrough at a temperature of 52°F.

The test bottles (at a weight of approximately 33.7 grams and having avolumetric capacity of approximately 317 cc) were then filled with acommercial carbonated soft drink, charged with CO₂ to a pressure of 75PSI absolute at 72°F and then stored at 100°F and 80% relative humidityfor 2 weeks.

The volume expansion or creep of the test bottles was then determined byplacing the bottles in a container of water with a fixed filling level.The volume of water displaced by the test bottles at the end of theperiod in excess of that displaced at the beginning of the period is ameasure of the volume expansion which occurred during the 2 weeks'storage time. It was determined by this creep test that the averagevolume expansion of the bottles made as indicated in this example was9.01% of the original volume.

Accordingly, applicant is proferred herein an apparatus and a processfor obtaining a thermoplastic container having a high resistance tocreep and deformation under pressure. In the above example, the interiorwall of the parison was directly cooled to a point just above its glasstransition temperature while the exterior wall is cooled below the pointand subsequently reheated by heat transfer. However, both surfaces mightbe reduced below the glass transition temperature and subsequentlyreheated during the "cooling" or delay cycle and such general procedurerepresents a most economical and rapid manner by which a thermoplasticmaterial may be reduced from its injection temperature to a more uniformorientation temperature. It is preferred that this more uniformtemperature at which expansion occurs be within a 50°F range aboveT_(G).

I claim:
 1. Apparatus for forming a biaxially oriented containercomprising:a. support means carrying an injection and expansion mold,said molds being opposite one another, said injection and expansionmolds having fluid passages therein for circulating fluid to cool theexterior walls of a parison formed in said mold to a first temperature,and b. core support means operatively associated with said support meanscarrying at least four cores radially extending from the support meansat 90° intervals and for indexing said cores sequentially to theinjection mold, a delay position, the expansion mold and to an ejectionstation, said cores having fluid passages therein for circulating fluidto cool the interior walls of said parison to a second temperaturehigher than the first temperature.
 2. An apparatus for forming abiaxially oriented hollow article comprising the combination of:a.support means carrying an injection mold and an expansion mold, saidinjection mold having a fluid cooling passage therein for circulating afluid at a first temperature; b. core support means operativelyassociated with said support means carrying a core for sequentiallyindexing the core in juxtaposition with the injection and expansionmolds and the core having a fluid cooling passage for circulating afluid at a second temperature higher than the first temperature and thecore support having fluid expansion passages therein for cooling andexpanding a parison; and c. means operatively associated with saidsupport means to index the parison to and maintain the parison at a timedelay station to permit the parison formed on said core to redistributeits heat from the interior of the parison throughout the parison priorto indexing the core into juxtaposition with the expansion mold.